The giant green anemone (Its scientific name is Anthopleura
xanthogrammica, if you're interested) is common along the Pacific
coast of North America, from Alaska all the way to Panama. So, if you
looked in the right place (tide pools and deep channels, on rocky shores)
in Oregon, you'd probably be able to find some. The giant green anemone
can grow to be larger than a foot in diameter, and can live for several
decades. I've attached a couple of pictures I found.

When I looked around, I found that these anemones like to eat lots
of things, including crabs, sea urchins, and small fish- basically
whatever they can catch. But most of all these guys like to eat mussels:
they're usually found around mussel beds, and they eat mussels that get
dislodged from the beds and swept to them by water currents. Anemones,
which are very similar to corals, are a class of animals (called
Anthozoans) in the phylum
Cnidaria, which also includes Jellyfish. Cnidarians
share with jellyfish something called
nematocysts, or "stinging cells".
Anemones have stinging cells on structures called acontia, which look
like little white threads on their tentacles. When they capture prey
they wrap their tentacles and acontia around it. The nematocysts
discharge, releasing a toxin into the prey. Once the prey is sufficiently
subdued, they can take it into their mouth at their leisure. The giant
green anemone also has little symbiotic algae cells that live inside cells
of the digestive layer. In exchange for a place to live, and a little
carbon dioxide, these algae release small amounts of sugars into their host
anemone.

Because these anemones can get so big, and live so long, I don't
think there's a lot of predation on them. I have been able to find
references to them being fed on by sculpins (a type of small fish), sea
snails, and sea spiders- I've attached a picture of each.